Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HETEROGENEOUS CATALYST
BACKGROUND OF THE INVENTION
The invention relates to a heterogeneous catalyst. The catalyst is especially
useful in
.. a process for preparing methyl methacrylate from methacrolein and methanol.
Heterogeneous catalysts having noble metals concentrated in an outer region of
the
catalyst are known, see, e.g., U.S. Pat. No. 6,228,800. However, there is a
need for larger
catalyst particles with noble metals more highly concentrated near the
surface.
SUMMARY OF THE INVENTION
The present invention is directed to a heterogeneous catalyst comprising a
support
and a noble metal, wherein said catalyst has an average diameter of at least
200 microns and
at least 90 wt% of the noble metal is in the outer 50% of catalyst volume.
The present invention is further directed to a catalyst bed comprising the
catalyst.
DETAILED DESCRIPTION OF THE INVENTION
All percentage compositions are weight percentages (wt%), and all temperatures
are
in C, unless otherwise indicated. A noble metal is any of gold, platinum,
iridium, osmium,
silver, palladium, rhodium and ruthenium. More than one noble metal may be
present in the
catalyst, in which case the limits apply to the total of all noble metals. The
"catalyst center"
is the centroid of the catalyst particle, i.e., the mean position of all
points in all coordinate
directions. A diameter is any linear dimension passing through the catalyst
center and the
average diameter is the arithmetic mean of all possible diameters. The aspect
ratio is the
ratio of the longest to the shortest diameters.
Preferably, the support is a particle of a refractory oxide; preferably y-, 6-
, or 0-
alumina, silica, magnesia, titania, zirconia, hafnia, vanadia, niobium oxide,
tantalum oxide,
ceria, yttria, lanthanum oxide or a combination thereof; preferably y-, 6-, or
0-alumina.
Preferably, in portions of the catalyst comprising noble metal, the support
has a surface area
greater than 10 m2/g, preferably greater than 30 m2/g, preferably greater than
50 m2/g,
preferably greater than 100 m2/g, preferably greater than 120 m2/g. In
portions of the
catalyst which comprise little or no noble metal, the support may have a
surface area with
less than 50 m2/g, preferably less than 20 m2/g.
Preferably, the aspect ratio of the catalyst particle is no more than 10:1,
preferably
no more than 5:1, preferably no more than 3:1, preferably no more than 2:1,
preferably no
more than 1.5:1, preferably no more than 1.1:1. Preferred shapes for the
particle include
spheres, cylinders, rectangular solids, rings, multi-lobed shapes (e.g.,
cloverleaf cross
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section), shapes having multiple holes and "wagon wheels," preferably spheres.
Irregular
shapes may also be used.
Preferably, at least 90 wt% of the noble metal(s) is in the outer 40% of
catalyst
volume (i.e., the volume of an average catalyst particle), preferably the
outer 35%,
preferably in the outer 30%, preferably in the outer 25%. Preferably, the
outer volume of
any particle shape is calculated for a volume having a constant distance from
its inner
surface to its outer surface (the surface of the particle), measured along a
line perpendicular
to the outer surface. For example, for a spherical particle the outer x% of
volume is a
spherical shell whose outer surface is the surface of the particle and whose
volume is x% of
the volume of the entire sphere. Preferably, at least 95 wt% of the noble
metal is in the
outer volume of the catalyst, preferably at least 97 wt%, preferably at least
99 wt%.
Preferably, at least 90 wt% (preferably at least 95 wt%, preferably at least
97 wt%,
preferably at least 99 wt%) of the noble metal(s) is within a distance from
the surface that is
no more than 15% of the catalyst diameter, preferably no more than 10%,
preferably no
more than 8%, preferably no more than 6%. Distance from the surface is
measured along a
line which is perpendicular to the surface.
Preferably, the noble metal is gold or palladium, preferably gold.
Preferably, the average diameter of the catalyst particle is at least 300
microns,
preferably at least 400 microns, preferably at least 500 microns, preferably
at least 600
microns, preferably at least 700 microns, preferably at least 800 microns;
preferably no
more than 30 mm, preferably no more than 20 mm, preferably no more than 10 mm,
preferably no more than 5 mm, preferably no more than 4 mm. The average
diameter of the
support and the average diameter of the final catalyst particle are not
significantly different.
Preferably, the amount of noble metal as a percentage of the noble metal and
the
support is from 0.2 to 5 wt%, preferably at least 0.5 wt%, preferably at least
0.8 wt%,
preferably at least 1 wt%, preferably at least 1.2 wt%; preferably no more
than 4 wt%,
preferably no more than 3 wt%, preferably no more than 2.5 wt%.
The catalyst of this invention is useful in a process for producing methyl
methacrylate (MMA) which comprises treating methacrolein with methanol in an
oxidative
esterification reactor (OER) containing a catalyst bed. The catalyst bed
comprises the
catalyst particles and is situated within the OER that liquid flow may occur
through the
catalyst bed. The catalyst particles in the catalyst bed typically are held in
place by solid
walls and by screens. In some configurations, the screens are on opposite ends
of the
catalyst bed and the solid walls are on the side(s), although in some
configurations the
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catalyst bed may be enclosed entirely by screens. Preferred shapes for the
catalyst bed
include a cylinder, a rectangular solid and a cylindrical shell; preferably a
cylinder. The
OER further comprises a liquid phase comprising methacrolein, methanol and MMA
and a
gaseous phase comprising oxygen. The liquid phase may further comprise
byproducts, e.g.,
methacrolein dimethyl acetal (MDA) and methyl isobutyrate (MIB). Preferably,
the liquid
phase is at a temperature from 40 to 120 C; preferably at least 50 C,
preferably at least 60
C; preferably no more than 110 C, preferably no more than 100 C. Preferably,
the
catalyst bed is at a pressure from 0 to 2000 psig (101 kPa to 14 MPa);
preferably no more
than 2000 kPa, preferably no more than 1500 kPa. Preferably, pH in the
catalyst bed is
from 4 to 10; preferably at least 4.5, preferably at least 5; preferably no
greater than 9,
preferably no greater than 8, preferably no greater than 7.5, preferably no
greater than 7,
preferably no greater than 6.5. Preferably, the catalyst bed is in a tubular
continuous reactor
or a continuous stirred tank reactor.
Preferably, the catalyst is produced by precipitating the noble metal from an
aqueous
solution of metal salts in the presence of the support. Preferred noble metal
salts include
tetrachloroauric acid, sodium aurothiosulfate, sodium aurothiomalate, gold
hydroxide,
palladium nitrate, palladium chloride and palladium acetate. In one preferred
embodiment,
the catalyst is produced by an incipient wetness technique in which an aqueous
solution of a
suitable noble metal precursor salt is added to a porous inorganic oxide such
that the pores
are filled with the solution and the water is then removed by drying. The
resulting material
is then converted into a finished catalyst by calcination, reduction, or other
treatments
known to those skilled in the art to decompose the noble metal salts into
metals or metal
oxides. Preferably, a C2-Ci8 thiol comprising at least one hydroxyl or
carboxylic acid
substituent is present in the solution. Preferably, the C2-C1s thiol
comprising at least one
hydroxyl or carboxylic acid substituent has from 2 to 12 carbon atoms,
preferably 2 to 8,
preferably 3 to 6. Preferably, the thiol compound comprises no more than 4
total hydroxyl
and carboxylic acid groups, preferably no more than 3, preferably no more than
2.
Preferably, the thiol compound has no more than 2 thiol groups, preferably no
more than
one. If the thiol compound comprises carboxylic acid substituents, they may be
present in
the acid form, conjugate base form or a mixture thereof. The thiol component
also may be
present either in its thiol (acid) form or its conjugate base (thiolate) form.
Especially
preferred thiol compounds include thiomalic acid, 3-mercaptopropionic acid,
thioglycolic
acid, 2-mercaptoethanol and 1-thioglycerol, including their conjugate bases.
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In another preferred embodiment, the catalyst is produced by deposition
precipitation in which a porous inorganic oxide is immersed in an aqueous
solution
containing a suitable noble metal precursor salt and that salt is then made to
interact with
the surface of the inorganic oxide by adjusting the pH of the solution. The
resulting treated
solid is then recovered (e.g. by filtration) and then converted into a
finished catalyst by
calcination, reduction, or other treatments known to those skilled in the art
to decompose
the noble metal salts into metals or metal oxides.
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EXAMPLES
Catalyst 778: "Egg Shell" Gold Catalyst
0.3088 g of sodium aurothiomalate was dissolved in 8.908 g of DI water with
stirring. Next,
10.0243 g of alumina (3.2 mm spheres, Norpro 5A6275, Lot No. 2016910048) which
had
previously been stored in a drying oven at 120 C were placed in a ceramic
crucible. The
aqueous gold salt was added drop-wise with periodic stirring using a spatula
until the incipient
wetness point of the support was reached.
The resulting material was dried for 1 hour at 120 C and then calcined in a
muffle furnace at
300 C (5 C/min ramp) for 4 hours. The resulting purple catalyst spheres were
then stored
in an amber vial until ready for use. Elemental analysis via NAA revealed the
following
elemental composition:
Description Au, wt% Na, ppm Cl, ppm
Catalyst 778 1.38 0.05 1000 30 340 20
Catalyst 780: Uniform Gold Catalyst (Comparative)
0.3918 g of sodium aurothiosulfate hydrate was dissolved in 9.0567 g of DI
water. A
10.0368 g sample of alumina (3.2 mm spheres, Norpro 5A6275, Lot No.
2016910048)
which had previously been stored in a drying oven at 120 C was placed in a
ceramic
crucible. The aqueous gold salt was added drop-wise with periodic stirring
using a spatula
until the incipient wetness point of the support was reached.
The resulting material was dried for 1 hour at 120 C and then calcined in a
muffle furnace
at 300 C (5 C/min ramp) for 4 hours. The resulting purple catalyst spheres
were then
stored in an amber vial until ready for use. Elemental analysis via NAA
revealed the
following elemental composition:
Calcination Calcination
Catalyst Temp Time
ID Description Au Precursor ( C) (hrs)
1.5wt% Au, sodium
778 3.2mm A1203 aurothiomalate (I) 300 4
1.5wt% Au, gold (I) sodium
780 3.2 mm A1203 thiosulfate hydrate 400 4
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EDS scans of the catalysts were done with the following results.
Catalyst 780 (comparative)
wt% Au Distance in microns from Volume % of the catalyst
outer edge of the catalyst
95 750 85%
90 680 81%
The data for Catalyst 780 show that 90 wt% of the gold is within 680 microns
of the outer
edge, corresponding to the outer 81% of catalyst volume; and that 95 wt% is
within 750
microns, or the outer 85%.
Catalyst 778
wt% Au Distance in microns from Volume % of the catalyst
outer edge of the catalyst
95 75 13.4%
90 70 12.6%
The data for Catalyst 778 ("egg-shell") show that 90 wt% of the gold is within
70 microns
of the outer edge, corresponding to the outer 12.6% of catalyst volume; and
that 95 wt% is
within 75 microns, or the outer 13.4%.
Catalyst Testing
Catalysts were evaluated in a continuous fixed-bed reactor operated in trickle
flow mode.
In each case, approximately 0.5 g of catalyst was mixed with silicon carbide
grit to ensure
uniform wetting. The catalyst bed was sandwiched between layers of glass
beads. The
reactor was operated at 60 C and 160 psig (1200 kPa) with an inlet oxygen
composition of
6 (achieved with 20 sccm air and 50 sccm He) or 21 mol% 02 at a gas flow rate
of 70 sccm.
Liquid feed (10 wt% methacrolein in methanol) was introduced at a flow rate of
0.07
mUmin. Performance over time, MMA rate as a space time yield, and MIB content
(ppm
on a 100% MMA basis) are shown in the table below.
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Oxygen level Space-Time Yield Product MIB
on a
fed to reactor lmol
MMA/kgcat/hrl 100% MMA basis
lmol%1 [PPml
"Egg-shell" 6 4 504
metal loading
(#778) 21 4 423
Predominately 6 1 740
uniform metal
loading (#780) 21 2 405
(comparative)
The data show that at high oxygen levels formation of byproduct MIB is low.
However, at
low oxygen levels, such as those that exist towards the end of a catalyst bed
where the
oxygen is depleted, the inventive "egg-shell" catalyst provides a greatly
reduced level of
MIB. The space-time yield for the inventive catalyst is superior at both
oxygen levels, but
especially at low oxygen level.
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